From what you've said, it sounds like your tank isn't cycled.
You need to get a liquid test kit and immediately begin testing your water. After a week with no water changes in an uncycled tank means your ammonia levels could be very dangerously high. (You might want to go ahead and do a partial water change immediately as a precaution if you don't have ready access to purchasing test kits.)
It should take about a month to 6 weeks for a new tank to cycle. The last time I did a fish-in cycle (i.e. cycling the tank while live fish are in it), it was a single medium gold fish, and I couldn't go more than 3 days without the ammonia levels getting dangerously high.
The other option (that is too late for you now that you already have fish) is a fishless cycle. This is where you douse the tank with ammonia to simulate feeding fish in the tank.
In either case, a fishless or fish-in cycle allows the tank to build up nitrifying bacteria that will convert the ammonia waste (fish pee/poop) which is highly toxic to fish into fertilizer. The purpose of long term water changes is to remove the fertilizer before it either becomes so concentrated that its dangerous to the fish, or it feeds too much algae growth.
But until your tank cycles (grows a colony of nitrifying bacteria), you need to be doing frequent water changes to remove the dangerous ammonia.